This invention relates to equipment, systems, and methods used to deoxygenate seawater. More specifically, the invention relates to a subsea catalytic deoxygenation unit in a subsea water injection process plant.
Current seawater injection process plants include topsides treatment steps, with equipment for each step placed on a platform (see FIG. 1). The treatment steps include seawater filtration and biofouling control by way of chemical additions. The filtration takes place at mesh sizes ranging from coarse strainers (to exclude fish and sand) to reverse osmosis membranes (to exclude monovalent ions). The chemical additions include in situ generated hypochlorite (electrochlorination). A deoxygenation or deaeration step is then added topsides so all gases are removed from the seawater.
Some seawater injection process plants are being located subsea (see FIG. 2). However, deoxygenation or deaeration, if used, must occur topsides.
The deaeration equipment used topsides relies on mass transfer processes. The most common of these processes is vacuum deaeration. This process cannot be implemented subsea due to the hydrostatic head above the equipment (e.g. 3000 m of water).
The deoxygenation equipment can be a catalyst bed-based seawater deoxygenation unit to remove dissolved oxygen from seawater by reacting it with hydrogen (see e.g. FIGS. 3-5 and US 2014/0054218 A1 hereby incorporated by reference). This reaction occurs on the open areas of the catalyst bed.
Removal of oxygen from seawater has value to a well operator because seawater with oxygen is highly corrosive. If a well operator wants to inject seawater that has not been deoxygenated, the operator needs to use pipework or tubulars made of an expensive corrosion resistant alloy or lined carbon steel. Even lined carbon steel is not perfect. Any tools put into the well to service it can damage a polymer lining or coating, leading to carbon steel corrosion.
Seawater without oxygen—for example, having an oxygen content no more than about 20 to 50 parts per billion—is relatively benign. Therefore, the pipework conveying the injection water to the reservoir can be made of low cost carbon steel.